Guides
Plain-English explanations behind every tool on this site: what each concept actually means, how it works, and how to fix the common problems.
Last updated: August 29, 2026
SPF Records Explained
What an SPF record does, the 10-lookup limit, and how to publish one without breaking mail.
DKIM Explained
How DKIM signs outgoing mail with a key pair, and how to generate and publish your own.
MTA-STS & TLS-RPT Explained
How to stop SMTP's TLS downgrade weakness and monitor delivery encryption failures.
Why DNS Propagation Takes Time
What TTL actually controls, and why different resolvers show different answers.
CIDR & Subnetting Explained
How to read CIDR notation and calculate network, broadcast, and usable host ranges.
Reading an SSL/TLS Chain
What leaf, intermediate, and root certificates are, and what to actually check.
Mail & Web Ports Explained
What each mail/web port is for, and the difference between implicit TLS and STARTTLS.
HTTP Security Headers Explained
What HSTS, CSP, X-Frame-Options, and the rest actually defend against.
Subdomain Takeover Explained
How a dangling CNAME leads to takeover, and how to prevent and fix it.